How to Interpret Data for Overly Rich Air-Fuel Mixture?

[DelFrank]

When the air-fuel mixture is overly rich, the onboard computer becomes ineffective and cannot display data. The following methods can be used for diagnosis: 1. Immediate Engine Stalling: This method is based on the theory that during idle, if the vacuum hose is suddenly disconnected and the engine stalls immediately, it indicates a lean air-fuel mixture. 2. Smoother Engine Operation: If the engine runs more smoothly than before, with a significantly higher but unstable idle speed, it suggests a rich air-fuel mixture. 3. Minimal Engine Change: If the engine speed does not increase (or increases slightly), with an idle that is somewhat unstable but close to normal, it indicates a properly balanced air-fuel mixture.

[Nina]

To diagnose a rich air-fuel mixture, I typically use an OBD scanner to read the engine's data stream. Pay attention to the oxygen sensor's voltage reading—if it consistently stays above 0.5V, the mixture might be too rich, as normal readings should fluctuate between 0.1V and 0.9V. Also, check the short-term fuel trim value. If it's negative, say -10% or lower, it indicates the system is reducing fuel supply to compensate for the rich condition. This could be caused by a clogged air filter or excessive fuel pressure, which I've encountered in my own car. Cleaning the filter or inspecting the fuel pump often resolves the issue. Ignoring it long-term can significantly increase fuel consumption and emissions, costing more money and harming the environment. Regular checks with simple tools can prevent major problems.

[RowanRose]

When diagnosing rich fuel mixture conditions, I prefer to directly monitor the scanner's live data stream. A significantly negative short-term fuel trim value is a key indicator—if it consistently drops to -15%, it confirms the mixture is too rich. Another critical point is the front oxygen sensor's output; if it remains steadily high without fluctuation (e.g., persistently displaying 0.6V), a rich condition is undeniable. During DIY repairs, I've found vacuum leaks or throttle body carbon buildup to be common culprits. After cleaning, data returns to normal, fuel consumption stabilizes, and the car regains its pep.

[ONorah]

I don't think the rich mixture data is hard to read. Just connect an OBD device to the car and check for high oxygen sensor values and large long-term fuel trim negative values. This is usually caused by excessive fuel injection or faulty air sensors, leading to black smoke emissions and a sharp increase in fuel consumption. Quick detection can extend engine life.